Typically, a viscose rayon fiber is manufactured by allowing a material pulp to react with alkali and carbon disulfide and dissolving the resulting matter in sodium hydroxide as alkali xanthate so that a spinning process is carried out with the cellulose being coagulated and regenerated.
The regenerated cellulose fiber typically represented by such viscose rayon has been used desirably for a long time as an artificial fiber having features close to natural fibers, such as a superior hygroscopic property. In order not only to prepare a fiber having features close to cotton or used as a substitute for natural fibers, but also to add new features thereto, various attempts have been made.
With respect to the modifying method for rayon, conventionally attempts to mix natural protein or a protein derivative into viscose and carry out a spinning process to produce a blended fiber have been made for a long time. The objective of these attempts is to allow the cellulose to have animal-based properties, and consequently to provide the viscose fiber with a dyeing property like that of dyes for wool and texture like that of wool. In this case, however, when protein is mixed in viscose, the protein is hydrolyzed by its strong alkaline property to make its own spinning original solution unstable, making it difficult to carry out a practical production in a uniform, stable manner.
In order to solve the above problem, a fiber in which protein (milk casein) is chemically bonded to cellulose has been proposed (Non-Patent Document 1). In this Non-Patent Document 1, a reaction product between milk casein and epichlorohydrin is mixed in viscose, and the cellulose is graft-polymerized by milk casein through epichlorohydrin by utilizing the high alkaline property of the viscose, and a detailed examination is made so as to carry out a spinning process in the middle of the reaction. In this case, however, the spinning original solution is gelatinized by the successively produced graft polymer to make the spinning process impossible, or to cause an insufficient dissolution of casein itself unless the alkali concentration is raised. Moreover, this problem also causes the hydrolysis of protein to accelerate and progress to an amino acid level, with the result that a severe limitation to the reaction time is required, making it difficult to carry out a practical production in a uniform, stable manner.
Attempts have also been made to modify protein into a resin-like material by utilizing a compound, such as acrylnitrile, acrylamide, ethylenediamine and melamine (see Patent Documents 1 and 2). In these methods, protein is only allowed to become one component forming the resin, and is modified greatly. In these cases also, a substantial amount of alkali has to be used for dissolving or dispersing the selected protein (casein) in the same manner as the above-mentioned process. Moreover, upon modifying into a resin-like material, the viscosity needs to be controlled to cause very complicated processes, failing to provide a practical production.
A technique has been proposed in which protein to be blended is mixed into cellulose without causing deterioration such as a reduction in the molecular weight of protein to that of oligomers or amino acids due to influences such as hydrolysis strongly exerted during manufacturing processes (Patent Document 3). In Patent Document 3, wool protein is skillfully adjusted so as to be alkali soluble and acidically coagulated, and the protein is preliminarily subjected to a crosslinking treatment by using a crosslinking agent so that the protein is not dissolved even in an alkaline spinning original solution. However, although the technique of Patent Document 3 is suitable for production of staple fibers, when it is applied to production of filaments in which protein as its yarn state is coagulation-regenerated and used for a long time up to the last process, it becomes difficult to produce filaments having uniform fineness and strength, and the problems of yarn disconnection and the like are also caused; consequently, this technique is not suitable for the production of filaments. Moreover, since the technique of Patent Document 3 needs to separately acquire specific protein components that are alkali soluble and acidically coagulated, another problem is the resulting high manufacturing costs.    Patent Document 1: Japanese Patent Application Publication Sho. 35-11458    Patent Document 2: Japanese Patent Application Publication Sho. 38-18563    Patent Document 3: Japanese Patent Application Laid-Open No.    Non-Patent Document 1: SEN-I GAKKAISHI, 1969, Vol. 25, p(24) to p(34), P 286 to P296